1. Field of the Invention
The present invention relates to turbines and, more particularly, to the vibration damping of turbine blades thereof.
2. Description of the Related Art
Turbines are commonly used to provide power to pump fluids, move vehicles, or generate electricity. The main power-producing component of a turbine is the turbine blade. Turbine blades are aerodynamically shaped vanes connected to the perimeter of a disk that rotates on a shaft. The blades are shaped so that, when a driving fluid passes over the surface, a force is generated causing the disk to rotate. They are usually manufactured as separate components that are subsequently attached to the disk by various means. Recently, however, turbine blades have been machined as integral parts of the disk. This one-piece integral blade/disk design is commonly referred to as a blisk.
During operation, turbine blades are subjected to alternating fluid forces that can cause high cycle fatigue failure, particularly if the frequency of the alternating force coincides with one of the natural vibration frequencies of the blade. In many instances, vibration dampers have been used to reduce the magnitude of the dynamic stresses, thereby increasing operational life. Most turbine blade vibration dampers consist of small metallic pieces that form a rubbing contact between two adjacent blades. Blade vibration causes motion at the blade/damper interfaces resulting in energy dissipation by friction. Since blisks consist of a single piece with no joints to dissipate vibration energy, they are particularly sensitive to operation near the natural frequencies of the blade/disk system. Turbine blades are designed to avoid primary resonant points but it is impossible to prevent this operation at all of the many blade natural frequencies. Therefore, additional damping must be provided to reduce resonant response of the blade/disk system.
Previous attempts to limit dynamic stresses within turbine blades have been disclosed in the patent literature. For example, U.S. Pat. No. 5,232,344, issued to Y. M. El-Aini, discloses a twisted hollow fan or compressor airfoil blade that extends radially from the rotor shaft. It has a plurality of internal chambers, each one bounded by the blade skin on two sides. A slug is located within at least one of these chambers, with the slug under the influence of centrifugal force in contact with the outboard section and also with one of the skins. It is in contact with the skins at two transversely spaced locations so that friction occurs between the two components.
U.S. Pat. No. 5,498,137, issued to Y. M. El-Aini et al., discloses a rotor blade for a turbine engine rotor assembly comprising a root, an airfoil, a platform, and apparatus for damping vibrations in the airfoil. The airfoil includes a pocket formed in a chordwise surface. The apparatus for damping vibrations in the blade includes a damper and a pocket lid. The damper is received within the pocket between an inner surface of the pocket and the pocket lid. The pocket lid is attached to the airfoil by conventional attachment apparatus and contoured to match the curvature of the airfoil.
U.S. Pat. No. 5,820,343, issued to R. J. Kraft et al., discloses a rotor blade for a rotor assembly that includes a root, an airfoil, a platform, and a damper. The airfoil includes at least one cavity. The platform extends laterally outward from the blade between the root and the airfoil, and includes an airfoil side, a root side, and an aperture extending between the root side of the platform and the cavity within the airfoil. The damper, which includes at least one bearing surface, is received within the aperture and the cavity. The bearing surface is in contact with a surface within the cavity and friction between the bearing surface and the surface within the cavity reduces vibration of the blade.
U.S. Pat. No. 5,165,860, issued to A. W. Stoner et al., discloses a turbine blade with an internal damper that comprises an elongated member with a damping surface of discrete width in contact with an interior surface of the blade. This contact is continuous throughout a contact length greater than 50% of the effective radial length. The contact is in the direction having a radial component with respect to the axis of the rotor, preferably with the damper extending between 2 degrees and 30 degrees from the radial direction. This damping surface is the exclusive frictional contact between the damper and the blade.
U.S. Pat. No. 4,484,859, issued to G. Pask et al., discloses an airfoil having a hollow portion at its tip and an internal surface of the hollow portion extending across the direction of centrifugal force acting on the blade in operation. The damper consists of a weight carried adjacent to the internal surface and free to bear on the surface under the action of centrifugal force. Should the blade vibrate, sliding movement may take place between the weight and the surface whereby the vibration of the blade is reduced.
U.S. Pat. No. 5,407,321, issued to D. A. Rimkunas et al., discloses the use of an elongated spring-like damper element that is shaped in the cross section of a xe2x80x9cVxe2x80x9d or xe2x80x9cUxe2x80x9d and inserted through a hole formed on one end of the ends of an airfoil of a stator vane. The legs of the xe2x80x9cVxe2x80x9d or xe2x80x9cUxe2x80x9d shaped element are adapted to bear against the inner surface of the airfoil and provide damping through frictional loss during vibration.
U.S. Pat. No. 6,283,707, issued to K. Chin, discloses a damper for an airfoil blade that comprises an elongated member that is inserted within a core passage in the blade. The damper is retained in the blade at the end closest to the blade root with the remainder of the damper free to move relative to and within the passage. The damper comprises a resilient plate insert upon which there are provided at least two discrete, oppositely directed, contact regions that are arranged to frictionally engage the passage.
Another proposed damping arrangement is described in GB 2078,310. In this proposal a pin is introduced within a slightly off radial extending passage provided in the airfoil portion of a blade. The pin is retained at the blade root end while being free to slide within the passage. Vibration of the blade causes relative sliding movement of the pin within the passage. Friction generated by the sliding movement absorbs energy and reduces vibration of the blade. The damping provided by this arrangement is achieved by contact between the pin and an interior passage within the blade. The pin must be closely fit to the passage and oriented at an angle to the radial direction so that a component of centripetal acceleration will force the pin to contact the wall of the passage.
The present invention is a turbine blade assembly for a turbine assembly. In a broad aspect, the turbine blade assembly includes a turbine blade having a turbine blade damper cavity formed therein. A stranded wire cable is positioned within the turbine blade damper cavity and is maintained there during operation of the turbine blade assembly. The stranded wire cable reduces vibration of the turbine blade assembly during operation by dissipating energy through friction between the cable and internal surface of the blade that define the damper cavity and also through friction between individual wires that make up the stranded cable.
This invention minimizes turbine blade high cycle fatigue failures by adding damping to reduce dynamic stresses. In a preferred embodiment, the stranded wire cable is held in place by a cap on the outer portion of the hole. During blade vibration, use of the stranded wire cable has been shown to reduce vibration stresses by as much as a factor of 20.
Most turbine blade dampers consist of separate elements that span between two adjacent blades. They provide damping by friction during relative motion of the blades. These designs are not used when the blade and disk are machined as a single entity (blisk) because the blades cannot be individually removed to install the dampers. The present invention is compatible with blisk configurations since the damper is completely contained within a single blade and does not span between adjacent blades. It is not limited to blisks and can also be used in conventional turbines where the individual blades are mechanically attached to the disk.
Other objects, advantages, and novel features will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.